The Sennheiser HD660s are dynamic-driver open-back headphones selling at an RSP of 599€ in the EU.

The Unboxing experience is very basic as with all Sennheiser Headphones, they come in a simple cardboard box,
included accessories are a protective bag, one single ended & one balanced 4.4mm cable of decent size (1.8m length)

The built quality of the headphones is a typical Sennheiser built, most parts are made out of plastic but quality plastic
that should last for a very long time. The earpads are very comfortable and made out of a suede fabric.
Out of the box the headphones can be quiet uncomfortable due to the initial clamp force but it will loosen up over time so
that overall comfort is about average. Both cables provided are of a decent quality and it's quiet nice that both
single ended & balanced options are available right out of the box which gives plenty of options for amplifier pairing.

On the technical side the drivers used inside the 660s2 are dynamic drivers with on impedance of 300 Ohms and 104db sensitivity
so on paper not so easy but also not to hard to drive, however adequate amplification is required to get the best performance
possible out these drivers.

Speaking about amplification these high-impedance cans are an ideal partner for an OTL tube amplifier and that is where i
found they sounded the best, don't get me wrong when paired with solid state amps like the Luxsin X9 and the Aune s17 pro
the performance was not bad at all, but it lacked a bit of the magical flavor, airiness and warmness added from the tubes
of the Feliks Audio Echo mk2.

Sound Signature: The 660s2 have a well balanced, transparent and natural tuning with a slight elevation in the lowend, not a bass canon
but better bass than it's predecessor the 660s which sounded thin by compairison. The trebble is also tuned very well, not too bright
but still quiet detailed and enjoyable.

Comparisons: Against the older model the 660s there is a noticeable step up in overall performance without any doubt.
The major issue though is the asking price of 599€, comparing against the Aune AR5000 which is about half the price it is
difficult to fully recommend the 660s2 as although both having a slightly different tuning, from the technical point of view for me
both headphones are in the same tier with even better built and comfort in favor of the AR5000.

Final Word: At the asking price the Sennheiser 660S2 is not more than a solid option, it's not a bad headphone but i think
there are possibly other great options out there to consider. Though things change considerably when paired with the right amplifier
or if it goes on sale.